Power semiconductor devices are generally designed and fabricated to have as low power losses as possible during forward conduction and switching applications. The conduction power losses are normally directly proportional to the so-called forward voltage drop, which is the voltage that has to be supplied across the power device, in particular a blocking layer (as explained below) of the power device, to conduct a certain current. It is generally desirable to reduce the forward voltage drop of a power semiconductor device for reducing power losses.
Power semiconductor devices normally have a relatively thick and lowly doped region, referred to as blocking layer or even high voltage blocking layer. The thickness and the doping concentration of the blocking layer determine the breakdown voltage of the device, wherein an increase in thickness and/or a decrease in doping concentration increases the resulting breakdown voltage. The high voltage blocking layer contributes substantially to the forward voltage drop and thereby the power dissipation in the on-state of the power device (i.e. forward conduction).
In a silicon (Si) power device, the forward voltage drop may be reduced by conductivity modulation, wherein electrons and holes (negative and positive charge carriers, respectively) are injected into a high voltage blocking layer of the device and the resulting carrier plasma formed by the injected electrons and holes reduces the forward voltage drop.
A silicon carbide (SiC) power device can, thanks to the high critical field strength of SiC, have a relatively high doping concentration and thereby a relatively low resistivity in its high voltage blocking layer. Thus, many SiC power devices operate without conductivity modulation and have still rather low power losses. In particular, SiC bipolar junction transistors (BJTs) are useful as power switching transistors. A BJT comprises a collector, a base and an emitter wherein the collector and the emitter are usually made of a first type of semiconductor material, for example n-type, and the base is made of another type of semiconductor material, in the present example p-type. Generally, one of the figures of merit of a SiC BJT is its low forward voltage drop, referred herein to as the collector-emitter saturation voltage “VCESAT”.
However, for high power applications where voltages larger than 3 kV are required or even for other applications where, for example, an operation temperature of at least 200° C. and voltages larger than 1 kV are required, conductivity modulation in SiC devices would be beneficial for reducing power losses.
Due to the bipolar nature of a SiC BJT, conductivity modulation can be obtained in the collector region by a carrier plasma consisting of electrons and holes injected from the emitter and base regions. As a result, the series resistance of the collector region is reduced, thereby reducing the forward voltage drop VCESAT. However, conductivity modulation presents also the drawback that the switching properties of the SiC BJT becomes slower because of the time required for building up and removing the carrier plasma during switching.
Thus, there is a need for providing methods and devices that would enable designing SiC BJTs while overcoming, or at least alleviating or mitigating, some of the above mentioned drawbacks.